The current trend in optical transmission networks is toward longer transmission distances with reduced signal regeneration, and the use of remotely re-configurable optical add/drop multiplexers (ROADM) to bring signals on and off the network backbone. ROADMs can be reconfigured dynamically in a process that is transparent to users. Current transmission distances reach ranges of 600 to 10,000 kilometers. Dynamic, transparent, optical networking results in improved transport economics and flexibility, but creates new problems such as the introduction of optical transients associated with adding and dropping channels into an optical transmission path. Signal attenuation also occurs due to a variety of factors including scattering, absorption, and bending. To compensate for signal loss and/or degradation, optical amplifiers (OA) are typically placed at regular intervals along the optical transmission path. Optical amplifiers amplify an input optical signal without converting it into electrical form
Many optical communication systems, especially long-haul networks, are wavelength division multiplexed (WDM) or dense wavelength division multiplexed systems (DWDM). (In this application, the use of the abbreviation WDM refers to wavelength division multiplexed systems, dense wavelength division multiplexed systems, and the like.) Such systems incorporate multiple channels, each at a slightly different wavelength, that are transmitted over a single optical fiber. Typically, a WDM network carries up to 40 or 80 channels, each channel transmitting perhaps in the range of 10 to 40 gigabits/second. By using many channels, the amount of data that can be transmitted over a single fiber is greatly increased.
Typically, optical signals in a WDM system must be processed approximately every 80 km to 100 km to restore the signal quality. This processing typically involves the use of OAs. Optimally, OAs in such systems should amplify all wavelengths consistently. However, in operation, achieving such consistency with OAs is very difficult to realize. For example, over time, the amount of amplification tends to vary within each channel and as a whole. It is not always practical and can be costly to recalibrate or replace OAs whose efficacy has diminished.
OAs include rare earth doped fiber amplifiers such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers. An EDFA operates by passing an optical signal through an erbium-doped fiber segment, and “pumping” the segment with light from another source such as a laser. Similarly, Raman Amplification occurs when the transmission fiber is pumped at an appropriate wavelength or wavelengths while light at a given input wavelength is transmitted through the fiber. Raman Amplification functions by transferring energy from a powerful pump beam to an emitted signal beam which is then an amplified version of the weak input signal beam. In Raman Amplification, the amplified light is typically achieved at a wavelength that is longer than the pump wavelength. The difference between the pump wavelength and the associated, emitted, amplified wavelength is referred to as a “Stokes shift.” The Stokes shift for a typical silica fiber is approximately 13 THz. Raman amplifiers provide amplification of an optical signal without the need for a specially doped fiber, such as used in an EDFA.
Stimulated Raman Scattering (SRS) is the basis for all Raman amplification. However SRS can also be a side effect of normal transmission of optical signals through a fiber, and, as such, can be a source of spurious light that results in signal degradation.
As a result of various loss and degradation mechanisms, the power loss that occurs as light is transmitted over greater distances is greater at shorter wavelengths than longer wavelengths. This causes a “spectral tilt” or simply “tilt” whereby the amplitude of the output spectrum varies with greater wavelength. Thus, the longer the distance of a fiber optic cable, the more pronounced is SRS, resulting in an increase in the power tilt towards the shorter wavelengths, i.e., higher frequencies.
Yet another cause of signal degradation is crosstalk which occurs when a signal at one wavelength interferes with the signal at another wavelength.
A Raman amplifier with a single pump may fail to provide gain over the bandwidth required in some WDM optical systems. To achieve a broadband gain characteristic, a plurality of pumps may be utilized in a single Raman amplifier. However, the gain spectrum from each pump tends to overlap such that a multi-pump Raman amplifier typically exhibits some variation in gain over the spectral range of the amplifier, known as “ripple.” This disparity in imparted gain can negatively affect signal quality and thus, maximum throughput (i.e. the maximum amount of data per time that is delivered).
Further degradation of the signal quality occurs when the polarization changes. In a typical optical network, each wavelength travels at two polarizations. Typically, the polarization of the signal changes slightly due to the above-described effects on the signal as well as when the signal is amplified in an OA or passes through a ROADM. An amplification span, the distance between one OA and another OA in an optical network, is typically about 80 km to 100 km. A fiber optic link in a fiber optic network can run thousands of kilometers. Therefore, many OAs are often needed. The slight changes in polarization accumulate as the signal passes through multiple OAs or ROADMs. When the polarization changes, the amount of power loss and amplification also changes, thus, further degrading the signal quality.
Another problem occurs when there is a fiber optic cut or loss in signal. In a dynamic optical network, a ROADM typically incorporates signals coming from multiple fiber optic cables or links into a single cable or link. These fiber optic links may originate at locations distant from each other, meet at the ROADM, and continue to propagate along a single fiber optic link. For example, an uplink fiber optic cable may be using 20 channels. Another uplink fiber optic cable may use another 20 channels. The uplink fiber optic cables meet at a ROADM and 40 channels are propagated through a common length of fiber optic cable. When the data channels transmitted over the links reaches the intended destination a ROADM separates such channels from the link. However, channels that continue on to another destination may be propagated over a new fiber optic link. This is often done to save money and add flexibility to a network as less fiber optic cables are needed and channels can be added or dropped when necessary. If one of the uplink cables is cut, there is a sudden drop in power. Conversely, if more channels are added to the network, there is a sudden gain in power. However, the OAs on the common length of fiber optic cable continue to amplify as if the power remained constant. The fast power variation causes large power fluctuations for each remaining channel. After passing through multiple OAs, a power surge occurs in the remaining channels whereby the signal strength of each channel increases at an increasing rate. This saturation effect results in increased and less predictable Raman Scattering, tilt, and polarization shifting. These effects cause the signal to be degraded and the performance of the network, such as the throughput, to decrease.
In recent years, many efforts have been made to deal with the above-described problems. U.S. Pat. No. 6,275,313 to Myron discloses that spectral distortion or Raman Scattering increases linearly with respect to input power. By keeping total input power constant through injecting one or more control signals, Raman Scattering can be predicted and pre-compensated for with an OA.
U.S. Patent Publication 2002/0044317 to Gentner, et al, expands the method disclosed in the above mentioned U.S. Pat. No. 6,275,313 by introducing a fast SRS tilt transient control system and a slow SRS static tilt control system. When adding or removing channels to a WDM, the overall power is measured and the power of the control signals are adjusted to make the total input signal power a constant. But this method runs the risk of introducing a severe four-wave-mixing (FWM) nonlinear penalty due to the high powers in the control signals. FWM is an intermodulation distortion in optical systems formed by the scattering incident photons.
U.S. Patent publication 2006/0127086 to Frankel and U.S. Patent Publication 2004/0131353 to Cannon, et al. provide methods of detecting a loss of signal and providing the signal from the other direction so as to maintain the same power within the optical link. This method complicates network design.
Krummrich, et al. (“Experimental investigation of compensation of Raman-induced power transients from WDM channel interactions,” IEEE Photonics Technology Letters, Vol. 17, no. 5, pp. 1094-1096, May 2005), U.S. Patent Publication 2004/0001710 to Peeters, et al., and U.S. Pat. No. 7,142,356 to Zhou disclose methods of using a dynamic gain tilt compensator (DGTC) to adjust the signal based on measurements of tilt. The DGTC-based method has the advantage of fast control speed and simple network management lacking in other prior art methods of transient control. However, greater precision in adjustment to optical signals is needed.
Thus, while prior art DGTC systems are fairly effective in recovering the original signal, a significant uncertainty in the calculation of SRS gain/loss still exists. There remains a long felt and unsolved need to find a method of reducing the sensitivity of the Raman gain spectrum measurement to polarization-related events.